Electric initiator



Feb. 7, 1967 c. M. SLOUGH 3,302,571

ELECTRIC INITIATOR Original Filed May 15. 1961 INVENTOR CARLTON M.SLOUGH ATTORNEY United States Patent "ice 3,302,571 ELECTRIC INITIATORCarlton M. Slough, Henrico County, Va., assignor to Texaco ExperimentIncorporated, Richmond, Va., a corporation of Virginia Continuation ofapplication Ser. No. 110,012, May 15, 1961. This application May 11,1964, Ser. No. 366,238 5 Claims. (Cl. 10270.2)

This application is a continuation of applicants copending applicationSerial No. 110,012, filed May 15, 1961, for Electric Initiator, and nowabandoned, and assigned to the same assignee as the present invention.

This invention relates to improvements in electric firin-g initiatingdevices and, more particularly, to electric initiators which are highlyresistant to premature firing by electromagnetic wave energy in theradio frequency range.

The invention relates to electrical firing initiating devices whichgenerally include a shell or casing in which is disposed an electricalresistance element or fuse wire which resistance element or fuse wire isin contact with a heat sensitive ignition composition or match headwhich, in turn, is embedded in or located adjacent to an explosivecharge.

Electrical firing initiating devices are commonly employed .to initiatevarious explosive compositions used as the primary igniter for reactionpropelled motors of the liquid or solid propellant types. In general,such electric initiators are designed to be actuated by direct currentand the art has long recognized the danger inherent in accidentaldischarge of such electric initiators by accidentally induced radiofrequency electromagnetic wave energy. More recently, this problem ofaccidental discharge of electric initiators has become more acute sinceradio frequency guiding systems and control means have been employed inreaction propelled space vehicles along with electrical firinginitiating devices.

It is, therefore, a primary object of the present invention to provide adirect current actuated electric initiator safe-guarded againstpremature or accidental initiation by radio frequency electromagneticwave energy induced therein.

A further object is to provide improved radio frequency electromagneticwave energy protected electric initiator without substantially reducingthe degree of sensitivity of the electric initiator to actuation bydirect current.

A further object is to provide such an initiator which is relativelysimple in construction and relatively inexpensive to manufacture.

In general, the invention consists of an electric initiator safeguardedagainst premature initiation by radio frequency electromagnetic waveenergy comprising a fuse wire, a heat sensitive ignition composition inheat exchange relationship to the fuse wire, an electrical conductorhaving one end connected to one end of the fuse wire and the other endof the electrical conductor being connectable to a source of initiatingcurrent, an electrical conductive tubular member surrounding theelectrical conductor for an appreciable length in the direction towardthe initiating current source, and electrically and mechanicallyencasing the fuse wire and ignition composition and connected to theother end of the fuse wire, said tubular member being dimensioned to benon-transmitting of electromagnetic wave energy of frequencies up to atleast about 2 kilo-megacycles.

The invention will be more particularly described with reference to theillustrative embodiments thereof shown in the accompanying drawingswherein:

FIG. 1 is a sectional view of an electrical initiator embodying theprinciples of the present invention;

FIG. 2 is an enlarged transverse section substantially on line 22 ofFIG. 1;

3,302,571 Patented Feb. 7, 1967 FIG. 3 is an enlarged horizontalsectional view similar to that shown in FIG. 2 of a modified form of thepresent invention wherein the outer electrical conductive member isrectangular in plan; and

FIG. 4 is an enlarged horizontal sectional view similar to that shown inFIG. 2 of a modified form of the present invention wherein the innerconductor is provided with an electrical insulating jacket and thejacket is positioned against one wall of the outer electrical conductivemember or conduit.

Referring to the drawings, 10 generally designates the improved electricinitiator safeguarded against premature initiation by radio frequencyelectromagnetic waveenergy induced therein. The initiator includes anelectrical resistance heater element or conventional fuse wire 12. Thefuse wire 12 is in contact or in heat exchange relationship to a heatsensitive initiator composition 14. Further, as illustrated in thedrawings, a primer spot or match head 16 may be in contactingrelationship to the fuse wire 12 and provide the initial ignitioncomposition. A suitable primer spot may comprise, for example, mercuryfulminate or lead azide.

End 12' of the fuse wire 12 is electrically connected to one end of anelectrical conductor 18 which is illustrated in the drawings as acylindrical copper wire. The remote end 18 of the electrical conductor18 is adapted to be connected to a source of initiating current, notshown, which, as will be more fully appreciated hereinafter, is directcurrent or very low frequency alternating current.

The conductor 18, the fuse wire 12 and the ignition composition 14 areprovided with a tubular electrical conductive housing generallydesignated 20. The illustrated housing 20 includes a conductive tubularconduit 22 having a substantially greater diameter than the conductor 18and the conductor 18 in this form of the invention is maintained insubstantially equidistant spaced relationship from the inner wall 24 byone or more electrical insulating spacers 26. The length of the tubularconduit 22 of the conductive tubular member 20 is critical for thedistance illustrated in FIG. 1 wherein it surrounds conductor 18, asfurther described below.

A conventional coaxial nut connecter 28 is illustrated at the upper endof conduit 22. It is rotatable on the conduit and in electrical contacttherewith. The remote end 18' of the conductor may be terminated in anyof the standard coaxial fittings, i.e., pin, socket, button-contact,etc., not shown, which are well known in the art. The choice of aparticular termination will be dictated by the type of electricalconnections extending from connector nut 28 to the source of initiatingcurrent. The length and type of such connections extending from nut 28are not critical, and open wire, coaxial cable, printed circuits, or thelike, may be used.

Nut 28 provides a return connection path for the initiating currentwhich is carried to the other end of the fuse wire by means of theconductive housing 20 as described below. Thus, when connected incircuit, the entire initiator assembly 10 will be grounded, asschematically indicated, to shield the same against the entry of radiofrequency electromagnetic wave energy at all points other than 18, 28.

The electrical conductive tubular member 22 may be circular or polygonalin cross-section with an inside diameter or side dimensioned to benon-transmitting of electromagnetic wave energy of frequencies up to atleast about 2 kilomegacycles to insure adequate protection for theinitiator.

Premature initiation by radio frequency electromagnetic wave energy hasbeen found to occur when the raceway or the conduit carrying the fusewire conductors acts as a waveguide. Where such a raceway or conduit isdiscontinuous at any point, an entrance is provided for radio frequencywave energy and the conduit acts as a waveguide such that the energy ispropagated down the waveguide to the fuse wire. At certain frequencies,depending upon the length and cross-sectional dimensions of the conduitattached to the initiator and the physical size of the initiator case,the initiator case acts as a resonant cavity and the fuse wire a pickupprobe. Heating of the fuse wire and initiation of the ignitercomposition can, therefore, occur if sufficient electromagnetic waveenergy is present.

By proper selection of the dimensions of the tubular member 22, theconductive tubing can be made nontransmitting of electromagnetic waveenergy below a predetermined frequency. Thus any radio frequency inducedenergy present on the fuse wire conductors leading up to connector nut28 will be attenuated in accordance with the invention to prevent theaccidental actuation of fuse 12. For example, a quadrangular waveguide10 mm. along its greatest side has a cut-off frequency of 15kilo-megacycles; that is, below 15 kilomegacycles, the conductive tubewill not propagate a wave. For a circular waveguide having the samecutoff frequency, its inside diameter is 1.17 times the permissibleinside dimensions of a polygonal conduit providing a permissiblediameter of 11.7 mm. for a cut-off frequency of 15 kilo-megacycles.

The cut-off frequency for a given waveguide below which it will notpropagate a wave is determined by the formula where F is the lowercut-off frequency; a is equal to the major waveguide interiorcross-section dimension in centimeters; and C is equal to 3 times 10 cm.per second, or the velocity of light. The free-space wavelength Acorresponding to F is related thereto by the formula C= \F, andtherefore dimension a will be equal to V2) measured in centimeters.

A waveguide is a rather narrow band transmission system which produces arapid attenuation of electromagnetic wave energies that are below thecut-off frequency. Energies are usually negligible at distances alongthe waveguide of a length slightly in excess of the major transverseinterior dimension of a: /2)\ In order to provide an increased safetyfactor in accordance with the invention, however, a minimum length fortubular member 22 should be not less than two wavelengths. Thus, in thepreceding examples for a cut-off frequency of 15 kilo-megacycles, themm. wide quadrangular waveguide should have a minimum length of 40 mm.;and the circular waveguide, 46.8 mm.

The lower end of the tubular member 22 may house the fuse wire 12 andthe ignition composition 14; however, where the inside diameter of thetubular member 22 is not sufficiently large to contain sufficientignition composition, the lower end 28 of the tubular member 22 receivesa further electrical conductive housing portion 30. The housing portion30, which may be tubular, is provided with an electrical conductive wallor roof 32 having a central opening therein which is connected to thelower end 28 of the conductor 22. The tubular conductor 22 and the wallportion 32 of conductor 30 are welded, sweated or otherwise securedtogether so that radio frequency electromagnetic wave energy cannot leakinto the system at the joint.

End 34 of fuse wire 12 is electrically connected to the inner wall 36 ofthe conductive member 30 by an electrical conductor 38. Further, thelower end of member 36 is closed by an electrical energy shield 40preferably comprising a thin metal foil which is either burnable orrupturable upon ignition of the igniter composition 14. The foil issuitably soldered as at 42 to the wall of the conductor 30 to preventleakage of RF energy into the housing containing the fuse wire 12.Similarly, an initiation destructible electrical shielding foil 44 isprovided adjacent the upper surface of the initiator composition tocontain such composition. The extremities of the foil 44 are suitablyelectrically connected to the inner wall of the conductive housing 30with the joint therebetween suitably formed to prevent the entry ofradio frequency energy into the initiator proper. Further, asillustrated in the drawings, foil 44 is insulated from the conductor 18as at 45.

A further protection for the initiator against premature initiation byradio frequency electromagnetic wave energy may be provided by theproper choice of the diameter of the conductor 18 in relationship to theinside diameter of the tubular conductive member 22 whereby theimpedance of the system, when considered as a coaxial cable, may bevaried through a range of, for example, 42 to 150 ohms without loweringthe cut-off frequency of the tubular member 22. This provides a choiceof mismatch conditions between the coaxial line and the fuse wire whichfurther reduces the possibility of ignition of the igniter composition14 from radio frequency electromagnetic energy which might be induced onthe center conductor 18.

The impedance of a coaxial line (Z is:

Diameter, Current AWG Wire Size mm. Capacity, Z Ohms Amps.

With the conditions indicated above, if the fuse wire 12 has animpedance of 1.0 ohm, then a mismatch ratio of 147 and 120,respectively, is provided with a copper wire conductor 18 having an AWGwire size of 18 and 14, respectively, in combination with a copper outertube having an inside diameter of 11.7 mm. Such mismatch conditionsresult in both a lower power transfer and a lower efficiency of energytransfer providing better protection against accidental initiation ofthe ignition composition due to accidental radio frequency pickup on theelectrical conductive conduits. When it is required to initiate theelectrical igniter direct current is employed; therefore, the impedanceof the coaxial line system has substantially no effect as the impedanceis simply the direct current resistance of the fuse wire 12 in serieswith the direct current resistance of the center electrical conductor18.

Where the outer tubular conductive conduit 22 and the inner conductor 18are constructed of conductive materials other than copper, greater orlesser losses may be introduced depending on the conductivity of thematerial used.

FIG. 3 is an enlarged horizontal sectional view, similar to that shownin FIG. 2, of a form of the present invention wherein the outerelectrical conductive conduit 22 is rectangular in plan and dimension dis 10 mm. The inner conductive member 18 of the illustrated device has adiameter d of 1.0 mm.

Referring to FIG. 4 of the drawings, there is shown a further form ofthe present invention wherein the outer conductive member is illustratedas a cylindrical conduit 22" and the inner conductor 50 is provided withan electrical insulative coating or jacket 52 and the insulatedconductor assembly is positioned against the inner wall of the conduit22. This form of construction is particularly advantageous where longruns and/ or sharp bends are required in the system which would affectthe position of the center conductor in the forms of the invention shownin FIGS. 1 through 3.

It is particularly pointed out that the cut-off frequency is of primeimportance and the form of the invention illustrated in FIG. 4 exhibitssubstantially no loss of effectiveness as a protective device forelectric initiators even though this form of the device does not havethe added protection provided by mismatch conditions which can be builtinto the systems shown in FIGS. 1 through 3.

From the foregoing description, it will be readily appreciated by thoseskilled in the art that the present invention fully accomplished theaims and objects hereinabove set forth and that various modificationsmay be made in the disclosed forms of the invention without departingfrom the scope of the appended claims. For example, while theillustrated examples of the invention disclose devices having cut-offfrequencies of about kilo-megacycles, the principles of the inventionmay be utilized in devices having substantially higher or lower cut-offfrequencies. As hereinbefore set forth, according to the formula C co lthe smaller the outer conductive tube, the higher the cut-off frequency,and substantially the only limitations which would be imposed would bethe ability to make initiators of small physical size and the necessitythat the center conductor must have a firing current carrying capacity.At the other end of the range, it has been found to be desirable thatthe outer tubular member be dimensioned to be non-transmitting of atleast the standard broadcast frequency electromagnetic wave energy.Therefore, the outer tubular member should be dimensioned to benon-transmitting of electromagnetic wave energy of frequencies up to atleast about 2 kilo-magacycles and, preferably, up to at least 10kilo-magacycles.

I claim:

1. An electric initiator safeguarded against premature initiation fromradio frequency electromagnetic wave energy comprising a fuse wire, aheat sensitive composition in heat exchange relationship to the fusewire, an electrical conductor having one end connected to one end of thefuse wire and the other end of the electrical conductor beingconnectable to a source of initiating current, an electricallyconductive tubular member surrounding the electrical conductor andextending from a point adjacent said one end of the fuse wire a lengthequal to the length of the electrical conductor, said length being notless than two wavelengths, A at a cut-off frequency of F said tubularmember being dimensioned transversely such that F equals C/Za where aisequal to the major waveguide interior cross-section dimension incentimeters, C is equal to the velocity of light in centimeters persecond and F is equal to at least about 2 kilo-megacycles; conductiveelectrical shielding means encasing said fuse wire and said heatsensitive composition, said shielding means having one wall surfacemechanically and electrically joined to said tubular member, and atleast one further wall surface formed of initiation destructibleconductive material,

and electrical conductor means electrically connecting the other end ofthe fuse wire and said shielding means.

2. An electric initiator safeguarded against premature initiation fromradio frequency electromagnetic wave energy comprising a fuse wire, aheat sensitive ignition composition in heat exchange relationship to thefuse wire, an electrical conductor having one end connected to one endof the fuse wire and the other end of the electrical conductor beingconnectable to a source of initiating current, an electricallyconductive tubular member surrounding the electrical conductor andextending from a point adjacent said one end of the fuse wire a lengthequal to the length of the electrical conductor, said length being notless than two wavelengths, A at a cut-off frequency of P said conductivetubular member being circular in plan and dimensioned transversely suchthat F equals C/Za Where a is equal to the waveguide interior diameterin centimeters, C is equal to the velocity of light in centimeters persecond and F is equal to at least about 2 kilo-megacycles; conductiveelectrical shielding means encasing said fuse wire and said heatsensitive composition, said shielding means having one Wall surfacemechanically and electrically joined to said tubular member, and atleast one further wall surface formed of initiation destructibleconductive material, and electrical conductor means electricallyconnecting the other end of the fuse wire and said shielding means.

3. The invention defined in claim 2 wherein the impedance of theelectrical conductor and said tubular member is greater than theimpedance of the fuse wire.

4-. The invention defined in claim 3 wherein the tubular member has aninside diameter of 11.7 mm. and the said electrical conductor has adiameter of from about 1 to about 1.6 mm.

5. An electric initiator safeguarded against premature initiation fromradio frequency electromagnetic wave energy comprising a fuse wire, aheat sensitive composition in heat exchange relationship to the fusewire, an electrical conductor having one end connected to one end of thefuse Wire and the other end of the electrical conductor beingconnectable to a source of initiating current, an electricallyconductive tubular member surrounding the electrical conductor andextending from a point adjacent said one end of the fuse wire a lengthequal to the length of the electrical conductor, said length being notless than two wavelengths, A at a cut-off frequency of P said tubularmember being rectangular in cross-section and dimensioned transverselysuch that F equals C 2a where a is equal to the major waveguide interiorcross-section dimension in centimeters, C is equal to the velocity oflight in centimeters per second and P is equal to at least about 2kilo-megacycles; conductive electrical shielding means encasing saidfuse wire and said heat sensitive composition, said shielding meanshaving one wall surface mechanically and electrically joined to saidtubular member, and at least one further wall surface formed ofinitiation destructible conductive material, and electrical conductormeans electrically connecting the other end of the fuse wire and saidshielding means.

No references cited.

BENJAMIN A. BORCHELT, Primary Examiner. W. C. ROCH, Assistant Examiner,

1. AN ELECTRIC INITIATOR SAFEGUARDED AGAINST PREMATURE INITATION FROMRADIO FREQUENCY ELECTROMAGNETIC WAVE ENERGY COMPRISING A FUSE WIRE, AHEAT SENSITIVE COMPOSITION IN HEAT EXCHANGE RELATIONSHIP TO THE FUSEWIRE, AN ELECTRICAL CONDUCTOR HAVING ONE END CONNECTED TO ONE END OF THEFUSE WIRE AND THE OTHER END OF THE ELECTRICAL CONDUCTOR BEINGCONNECTABLE TO A SOURCE OF INITIATING CURRENT, AN ELECTRICALLYCONDUCTIVE TUBULAR MEMBER SURROUNDING THE ELECTRICAL CONDUCTOR ANDEXTENDING FROM A POINT ADJACENT SAID ONE END OF THE FUSE WIRE A LENGTHEQUAL TO THE LENGTH OF THE ELECTRICAL CONDUCTOR, SAID LENGTH BEING NOTLESS THAN TWO WAVELENGTHS, ACO, AT A CUT-OFF FREQUENCY OF FCO, SAIDTUBULAR MEMBER BEING DIMENSIONED TRANSVERSELY OF FCO, SAID FCO EQUALSC/2A WHERE A IS EQUAL TO THE MAJOR WAVEGUIDE INTERIOR CROSS-SECTIONDIMENSION IN CENTIMETERS, C IS EQUAL TO THE VELOCITY OF LIGHT INCENTIMETERS PER SECOND AND FCO IS EQUAL TO AT LEAST ABOUT 2KELO-MEGACYCLES; CONDUCTIVE ELECTRICAL SHILEDING MEANS ENCASING SAIDFUSE WIRE AND SAID HEAT SENSITIVE COMPOSITION, SAID SHIELDING MEANSHAVING, ONE WALL SURFACE MECHANICALLY AND ELECTRICALLY JOINED TO SAIDTUBULAR MEMBER, AND AT LEAST ONE FURTHER WALL SURFACE FORMED OFIITIATION DESTRUCTIBLE CONDUTIVE MATERIAL, AND ELECTRICAL CONDUCTORMEANS ELECTRICALLY CONNECTING THE OTHER END OF THE FUSE WIRE AND SAIDSHIELDING MEANS.